A capacitor is a device that stores energy in the form of charges of equal and opposite signs on two plates, separated by a non-conducting medium. The capacitance (C) of such a device is given by the following relation: ##EQU1## where q is the charge on each plate, V is the potential difference between the plates, A is the surface area of each plate, .epsilon. is the dielectric constant of the medium separating the plates, and d is the distance between the plates. Furthermore, the potential energy (U) stored in a capacitor is given by: ##EQU2##
These simple relations indicate that in order to have a high energy capacitor, one requires plates with high surface area and very close spacing. If two flat metal plates that each have an area of 2 m.sup.2 are separated by 1 mm in a vacuum, the associated capacitance is only 1.75.times.10.sup.-8 farads (F). Thus, the farad is a large value of capacitance. A more practical measure of capacitance is the microfarad (.mu.F).
Electrochemical capacitors store charge at the interface between a solid electrode and an electrolyte through physical-chemical interactions between the electrode and the ions in the electrolyte. Raistrick, Electrochemical Capacitors, in "Electrochemistry of Semi-conductors and Electronics Processes and Devices," Eds. J. McHardy and F. Ludwig, Noyes Publications, 1992 (incorporated herein by reference in its entirety), reviews the history and properties of electrochemical capacitors. Only a brief summary of the nature of electrochemical capacitors follows. Reference to Raistrick is recommended.
When ions are present in a system that contains an interface, there will be a variation in the ion density near that interface. When a potential difference is applied across two electrodes in a solution, a charge builds up on the surface of each electrode. For a metal electrode, this charge resides on a very thin layer (less than 0.1 .ANG. thick) at the metal surface. Due to electrostatic interactions, ions in the solution migrate to the electrode to counterbalance the charge on the electrode. The excess charge density at the interface can be ascribed to two parallel imaginary planes which contain opposite charge. These two planes are called the electric double layer. Each layer corresponds to a plate of the double layer capacitor. Some ions that can be specifically adsorbed on the surface of the electrode are associated with a plane called the inner Helmholtz plane (IHP) which represents the position of these ions. The solvated ions can only approach the surface to a distance referred to as the outer Helmholtz plane (OHP). Since either of these layers are extremely close to the surface (on the order of Angstroms, 10.sup.-10 m), typical capacitance values for these electrolytic capacitors are 10-50 .mu.F/cm.sup.2 of electrode surface. Thus, the capacitance of this system is on the order of 10.sup.7 times greater than that of a traditional parallel plate capacitor with the same surface area.
Electrochemical ultracapacitors, based on high surface area carbon electrode materials and a variety of electrolytes, have been the most studied and commercially developed systems. High porosity carbon, usually in the form of activated carbon or carbon black that is immersed in an aqueous or non-aqueous electrolytic solution, has a very high capacitance due to its large specific surface area. Tanahashi et al., "Electrochemical Characterization of Activated Carbon-Fiber Cloth Polarizable Electrodes for Electric Double-Layer Capacitors," J. Electrochem. Soc., 137: 3052-7 (1990), showed that an activated carbon fiber cloth (ACFC) electrode can yield a differential capacitance of 113 F/g as measured by cyclic voltametry. Other research has shown that the ACFC can achieve a specific capacitance of 20-32 F/g in solid state or organic electrolytes. Mayer et al., "The Aerocapacitor: An Electrochemical Double-Layer Energy-Storage Device," J. Electrochem. Soc., 140:446-51 (1993) developed an aerogel of carbon with a specific capacitance on the order of several tens of farads per gram. Most recent studies show that the specific capacitance of the carbon electrode can be as high as about 400 F/g.
The double-layer charging process may be regarded as taking place without complete charge-transfer to or from ionic species either in the solution or adsorbed at the interface. On the other hand, specifically adsorbed species undergo chemical interaction with the electrode and some degree of charge transfer between the metal or metal oxide and the adsorbate occurs. Some of these specifically adsorbed species undergo a Faradaic charge transfer process upon adsorption on the surface of the electrode. Examples of this type of reaction involve electrosorption of protons at noble metal surfaces (e.g., Platinum). Other reactions include reactions at thermally prepared RuO.sub.2 and IrO.sub.2 films of both a proton and an electron.
The differential capacitance associated with electrosorption processes is often called a "pseudocapacitance," C.sub.ads, which can be expressed as .differential.q/.differential.E=q.sub.0 (.differential..theta./.differential.E), where .theta. is the fractional surface coverage by the adsorbed species, q.sub.0 is the charge associated with the adsorption of a monolayer, and E is the potential applied on the electrode. This pseudocapacitance or "Electrosorption Capacitance" is reviewed in much greater detail in Raistrick.
Depending upon the materials, either the electric double layer or the electrosorption of ions at the interface, or both, can contribute to the storage of a tremendous amount of charge when a high specific surface area electrode is used. High surface area electrochemical capacitors can store higher charge than conventional capacitors.
Raistrick also graphically illustrated the relationship between specific power (kW/kg) and specific energy (kJ/kg) for conventional capacitors and batteries. While batteries have relatively large energy storage capability, their specific power output is relatively low. Thus, batteries are useful when sustained low power output is required. In contrast, conventional capacitors cannot store as much energy as batteries but can put out very high power levels for short times. Electrochemical capacitors present a hybrid between batteries and conventional capacitors in that they deliver higher peak power than batteries and yet have a higher energy storage capacity than conventional capacitors.
This attribute makes electrochemical capacitors potential candidates for electric vehicles and other devices that can provide sufficient power to meet the short-term heavy power demands often encountered by electric devices. Electrochemical capacitors can also serve as back-up energy sources in integrated electronics.
Two types of materials, carbon and oxides, are applied in electrochemical capacitors. Sohio Engineered Materials Company has produced a carbon-based capacitor that provides up to two farads per cubic inch of the device. This capacitor has 100 to 500 times the energy density of an aluminum electrolytic capacitor of similar capacitance and voltage. Other companies make carbon-based materials as well.
An electrochemical capacitor based upon oxide materials, namely RuO.sub.2, is marketed by Pinnacle. See, Bullard, et al., "Operating Principles of the Ultracapacitor," IEEE Transactions on Magnetics, 25:102 (1989). As noted by Raistrick, comparatively little research has focused on oxide systems, possibly because of the high cost of RuO.sub.2 which will limit the application of RuO.sub.2 electrochemical capacitors, even if electrosorption on the surface of RuO.sub.2 offers otherwise excellent charge storage capabilities.
Many transition metal oxides are electrochemically active and it has been envisioned that metal oxides can be used as electrodes in electrochemical capacitors, although Raistrick indicated that they are more justifiably considered battery materials, because the chemistry of these metal oxide materials has traditionally been thought to relate to the bulk of the materials, rather than to their surfaces. Examples include manganese oxide, nickel oxide, vanadium oxide, tungsten oxide, cobalt oxide, chromium oxide and molybdenum oxide. The charge densities of various oxide systems, calculated from the electrochemical reactions of these oxides, and based on their bulk electrochemical reaction in batteries are listed in the Raistrick review. These charge densities provide some indication of the energy-storage capability of a `capacitor` constructed from these materials.
There is one report of the use of a highly dispersed, thin electrode of a number of oxides, including VO.sub.x, IrO.sub.x, RuO.sub.x and NiO, and organic polymers, in a multilayer bipolar pulse power battery/capacitor configuration fabricated by vacuum and electrochemical deposition techniques (Rauh, D., "High Rate Solid Electrodes for Pulsed Power," in Proc. U.S. Army Workshop on Capacitors and Batteries for Pulse Power Applications, Laboratory of the U.S. Army, LABCOM, Asbury Park, N.J., Nov. 17-18 (1987)). However, Rauh noted the limited electronic conductivity over the entire potential range of xerogel-based materials, especially NiO, polypyrrole, and IrO.sub.2, and less so for RuO.sub.2.
Also, a patent to Elliot and Huff (U.S. Pat. No. 3,317,349; issued May 2, 1967) concerned the use of oxides in capacitors. Wang, et al., "Preparation of Nickel Oxide Films by Sol-Gel Process," J. Ceramic Society of Japan, 101:223-5 (1993) reported the production of a nickel oxide film through a sol-gel process using an organic solvent (i.e., ethylene glycol). Hu et al. (1993) demonstrated that a nickel oxide film composed of fine particles displays better electrochromic properties than a film with homogenous morphology, and suggested that the grain boundaries provide a channel to enhance surface redox reactions which enhance the electrochromic effect.
It would be desirable to produce and use an inexpensive electrochemical capacitor having the desired capacitance and pseudocapacitance properties to provide very high power output for short periods of time coupled with rapid and efficient recharge.